1. Field of the Invention
This invention relates to oscillator actuated hydraulic impulse devices and in particular oscillator actuated hydraulic impulse devices which are provided with means for controlling the stroke of the piston which strikes a boring tool.
2. Prior Art
Conventional hydraulic impulse devices in practical use have been of the self-actuating type in which reciprocating motion of the piston which strikes the boring tool is caused by the switching of hydraulic pressure in a double acting cylinder by means of a spool valve actuated by changes in the position of the piston. Such devices have suffered from several drawbacks. First, since the period of the spool is determined by the period of the reciprocating motion of the piston, the striking frequency and the striking energy cannot be varied independently of each other. In other words, if the hydraulic pressure supplied to the piston is reduced in order to decrease the striking energy, the striking frequency is also reduced due to a reduction in the velocity of the piston. Accordingly, it is difficult to obtain efficient boring which can be adapted to geological variation. Particularly, in the case of rotary percussion drills, it is necessary to maintain an optimum combination of drill RPM, striking frequency, striking energy, feed, etc., and to change the RPM of the drill in accordance with variations in the striking frequency in order to achieve efficient boring. Complicated and expensive control devices are required in order to accomplish this end.
Furthermore, some reports state that when the piston strikes the boring tool, the efficiency of the energy transmission to the rod is improved if the period of time (hereinafter referred to as the push time) during which the piston pushes against the boring tool after initial contact is made is increased to some extent. In conventional devices, however, this push time is almost non-existant.
Furthermore, since conventional devices are of the self-actuating type, a dead point exist. Therefore, the device may fail to start, depending upon the relative positions of the piston and the spool when the device is stopped. Accordingly, it is frequently necessary to attach a separate starter device in order to insure starting.
Furthermore, in such devices which are equipped with bladder type or diaphram type accumulators for the purpose of recovering energy from the return stroke of the piston and using it in the impulse stroke in order to increase efficiency, the striking frequency cannot be increased to any great extent due to problems in the accumulator response and durability.
In addition, conventional oscillator actuated hydraulic impulse devices use oscillation exciters which suffer from certain drawbacks when used in industrial equipment. Specifically, mechanical oscillation exciters which utilize an eccentric mass are excessively large in terms of structural size. Electrical oscillation exciters are easier to control and can achieve high oscillation frequency, but make it difficult to obtain a large power output. Devices which utilize a motor and air pressure generate excessive noise. Devices utilizing electro-hydraulic servo valves can achieve a large power output but are expensive and unsuitable for use in construction and mining machinery which is operated under harsh environmental conditions.
On the other hand, devices for generating oscillating pressure which utilize the self-excited oscillation of a hydraulic valve have been proposed. Oscillation exciters which utilize such devices as sources of pilot pressure have been able to compensate for previously mentioned drawbacks, but such devices are still unsatisfactory in some respects for use in percussion machinery such as hydraulic rock drills, hydraulic breakers, etc. Specifically, since the pressure variation of the oscillating pressure is small, this type of device cannot be used as a source of pilot pressure in machinery which is required to develop a large power output. Furthermore, if the device is designed such that the boring tool is struck at the point of maximum piston velocity in order to obtain a high striking energy, a considerable amount of time is required for the piston switching valve switch so that the piston begins its return stroke after striking the boring tool. This leads to an excessive increase in the amount of idle time during which the piston is in contact with the boring tool which in turn results in a decrease in efficiency rather than an increase.
Particularly, in cases where this type of device is used in a rotary percussion drill in which the bit is caused to revolve by means of a motor, etc., bit bradrasion becomes excessive. Furthermore, in order to achieve satisfactory efficiency, it is necessary to gradually store up energy during the return stroke of the piston by means of a spring (coil spring or accumulator, etc.) to release this energy in a rapid surge during the impulse stroke so that the piston is accelerated to a high velocity before it strikes the boring tool. For the above reasons, it is desirable that the time required for the impulse stroke of the piston be shorter than the time required for the return stroke. In conventional devices for generating oscillating pressure, however, the period of time during which a high pressure is maintained and the period of time during which a low pressure is maintained are approximately equal. Accordingly, the duration of impulse stroke and the duration of the return stroke are approximately equal.